1. Field of the Invention
This invention relates to an actuator for a wireline blowout preventer.
2. The Prior Art
U.S. Pat. No. 3,692,316 discloses an hydraulic actuator for a wireline blowout preventer which relies upon pressurized hydraulic control fluid to both open the blowout preventer and effect closure of the blowout preventer. U.S. Pat. No. 2,163,178 discloses a blowout preventer wherein hydraulic fluid is utilized to urge the preventer ram towards its closed position. However, the blowout preventer disclosed in U.S. Pat. No. 2,163,178 cannot be opened hydraulically. The rams of the preventors disclosed in U.S. Pat. Nos. 3,692,316 and 2,163,178 may be mechanically advanced towards their closed position and mechanically locked against movement towards their open position. However, if the preventers are to be closed hydraulically hydraulic control fluid pressure must exceed well pressure. A failure in the hydraulic control system and loss of hydraulic control fluid pressure would render these preventers exceedingly difficult to close against high well pressure.
Certain actuators do utilize the controlled fluid as a source of fluid for affecting operation of a valve. U.S. Pat. No. 1,854,058 discloses communicating well fluids upstream from the ram of the preventer to one side of the actuator piston. The well fluids urge the piston towards a position wherein the ram closes the preventer. The patent additionally discloses that a mechanical sprocket lever may advance the ram of the preventer towards its closed position and that ram closure may be assisted by a screw jack. Once the ram is closed, ram drift may be prevented by the screw jack. However, with the disclosed actuator, no automatic closure of the blowout preventer can be effected whenever well pressure exceeds hydraulic control pressure. Additionally, the disclosure only suggests that upstream well fluids assist the mechanical sprocket lever in urging the preventer rams towards their closed position. The rams and the actuator piston do not and cannot move independently of the sprocket lever. Finally, there is no hydraulic way to open the preventer disclosed in this patent.
U.S. Pat. No. 3,800,822 discloses a pilot valve controlled main valve. The pilot valve, when activated, opens communication between upstream well fluids and the actuator piston chamber. The pressure of the upstream well fluids thereafter moves the main valve to its closed position. There is no disclosed hydraulic means for opening the main valve. Neither are there any disclosed mechanical means for advancing the main valve towards its closed position and maintaining the main valve in its closed position. Finally, well fluids are not confined by the disclosed main valve, actuator and pilot valve combination. Opening of the main valve requires that well fluids be bled from the actuator piston chamber. Bleeding well fluids out of the system is not always possible and may be unsafe.
A "TYPE X OTIS ACTUATOR" as disclosed on page 4027 of the 1974-75 edition of the "COMPOSITE CATALOG OF OILFIELD EQUIPMENT AND SERVICES" is a pilot valve controlled actuator. One side of the actuator piston is always affected by upstream fluids. The pilot valve controls communication between the other side of the actuator piston and the upstream fluids. When the pilot valve is closed, the communicating passage is opened and the piston is pressure balanced. The main valve may be moved to its open position. However, since the piston is pressure balanced, mechanical means are required to move the main valve to its open position. When the pilot valve opens, the communicating passage closes. Opening of the pilot valve bleeds upstream fluids from the piston chamber. The required bleeding of fluids renders the actuator impractical for certain installations. Additionally, there are no disclosed mechanical means for assisting movement of the main valve member towards its closed position and for locking the main valve member in its closed position.
U.S. Pat. No. 4,087,073 discloses another pilot valve controlled actuator for a main valve. Normally, the pilot valve is closed and fluids upstream of the main valve affect only one side of the actuator piston. The piston in turn maintains the main valve opened. When the pilot valve opens, the piston becomes pressure balanced. A spring moves the piston to a second position wherein the main valve member closes the main valve. To reopen the main valve requires that the other side of the piston be bled of the upstream well fluids which have entered therein. Under certain circumstances the required bleeding of upstream fluids from the piston chamber is unsafe and cannot be performed. Since the actuator is pilot valve controlled, there is no automatic closure whenever well pressure exceeds control fluid pressure. Additionally, there are no net fluid forces on the piston during closure. Finally, there are no mechanical means for positively advancing the main valve member towards its closed position.